If you want to control where the mail gets delivered and which mailbox format is to be used, you can do this by setting

home_mailbox = /some/path

or

mail_spool_directory some/path

mail_spool_directory is an absolute path where all mail goes, while home_mailbox specifies a mailbox relative to the user's home directory. If the path ends with a slash ('/'), messages are stored in Maildir format (direcory tree, one message per file); if it doesn't, the mbox format is used (all mail in one file).

Examples:

mail_spool_directory = /var/mail (1)
home_mailbox = Maildir/ (2)

1) All mail will be stored in /var/mail, mbox format

2) Mail will be saved in ~/Maildir, Maildir format

Step 2: Edit /etc/rc.conf

Add postfix to the daemons list. Make sure you put it after the network has started.

DAEMONS=(syslog-ng network netfs crond postfix)

Step 3: Edit /etc/postfix/aliases

Namely, put a username in for the the following, substituting the correct username you desire (actual system account required) for USER